The invention relates to a toothed wheel unit with external toothing, preferably for heavy machinery gears and/or rapidly moving gears like turbomachinery gears, comprising a toothing carrier with an external toothing provided on a circumferential surface and a bearing journal or connection portion in at least one axial end region of the toothing carrier, joined with the toothing carrier.
Toothed wheel units of this type of interest are e. g. used in spur gears, as shown in BHS catalogue xe2x80x9cBHS-Stirnradgetriebe, einstufig, fxc3xcr hochtourige Anlagenxe2x80x9d (BHS spur gears, single stage, for high-speed installations) with masthead H-3/1-87.
In such gears, preferably the large spur toothed wheels have dimensions with diameters from e.g. 500 mm up to 1250 mm. Simultaneously the axial width of the large toothed wheels can be in the range of 200 to 600 mm in the case of spur toothing and single helical toothing, and in the range of 250 to 700 mm in the case of double helical toothing.
It is known to produce such toothed wheel units in full shaft construction. Full shaft construction means that a plate body is produced integrally with a shaft, the shaft projecting over the end surfaces of the plate body and serving for bearing. This full shaft construction has advantages. The stress due to the centrifugal force and the difference in heat between the toothing region and the central region is small. Furthermore, due to the massive construction, small variations in shape (constant volume) result e.g. in the case of stress by centrifugal forces.
However, the full shaft construction also presents some disadvantages. In order to illustrate these disadvantages, the way of producing such toothed wheel units in full shaft construction has to be outlined in brief. At first, a cast steel part is produced, which comprises the shaft and the plate body in integral construction. This case steel part is then forged and after that subjected to a metal cutting treatment, in order to define the final dimensions and preferably to provide the toothing. To reduce the casting and forging stresses and to optimize the texture of the material, various heat treatments are executed in between. Subsequently, carbonization is performed at approx. 900xc2x0 C. and after that it is hardened (case hardening).
The thus produced hardening strains can amount to considerable values, especially in case of larger parts, and cause problems.
Further problems are to a considerable degree due to the process of casting, in which a so-called segregation zone forms in the central region of the shaft and the plate body, in which impurities of the steel alloy and perturbations of the texture structure are present in high concentrations.
When the finished toothed wheel unit is finally commissioned, additional centrifugal force stresses and operation heat stresses occur, especially during rapid starting operation. These stresses can lead to uncontrollable stress conditions in the segregation zone.
In order to eliminate the negative effects of the segregation zone, which are the larger, the larger the volume of the entire toothed wheel unit is, it has already been thought of going over to a hollow shaft structure, preferably by reboring the segregation zone so that the aggregation of bad material in the interior of the shaft is removed. However, this reboring of the shaft has not allowed to solve all the problems. On the one hand, the stresses occurring due to the centrifugal force and the temperature differences are increased with respect to the full shaft structure so that the material can give way, when centrifugal forces occur in the region of the inner diameter, and thus, additional possibilities for the deformation of the plate body occur. On the other hand, the concentration of impurities and perturbations of the texture structure known as segregation persists in the regions adjacent to the bore. Furthermore, hardening strains cannot be excluded, because the hardening process is insufficient in the region of the bore.
Toothed wheel units have also been considered in which the plate body is shrunk on the shaft. The application of this structure is limited, because a disengagement of the plate body from its connection with the shaft body cannot safely be avoided due to high centrifugal force and heat loads. The tendency, therefore, still goes to an integral structure.
It is an object of the invention to construct a toothed wheel unit of the generic type in such a way that strain conditions, which are difficult to control, especially hardening strain conditions, are avoided or minimized.
To achieve the object it is proposed according to the invention that the toothing carrier is either provided with a toothing carrier casing, whose inner diameter is larger than the diameter of the bearing journal or the connection portion, or/and with a plurality of toothing carrier plates following each other in axial direction and joined with each other.
With the construction according to the invention, segregation zones can be avoided to a large extent or completely, as the concentration of material over a large volume hardly accessible to heat treatments from the outside is avoided. With the construction according to the invention, all volume regions of the components, i.e. the toothing carrier casing, the bearing journals, the connection portions and the toothing carrier plates, are relatively thin-walled so that, on the one hand, in contrast to large volume work pieces, they do not present comparable problems caused by the segregation zone and, on the other hand, external treatments act more homogeneously onto all volume regions.
It may happen that with the toothed wheel units according to the invention, individual components or regions of components are exposed to higher stresses than with the known toothed wheel units of the full shaft construction and the hollow shaft construction. However, it has turned out that the occurring stress distributions are easier to calculate, in particular because elements like tubular body plates lead to two-dimensional, possibly even linear stress conditions and cubically oriented radial stress profiles are avoided.
In the case of a tubular toothing carrier casing, the stress problem reduces to the problem of a thin-walled tube, a wall thickness of 150 to 200 mm being possible, when the toothing diameter is 900 mm and more. In case of smaller toothing diameters, the wall thickness is reduced corresponding to the ratio of the toothing diameters. This thin-walled tube is shorter than the length of the shaft in case of a full shaft construction. Due to the thinness of the wall, an almost uniform heat treatment of all volume regions can be expected on case hardening. Forging is also rendered easier and more favourable in view of the avoidance of stress conditions, inasmuch as the toothing carrier casing can be rolled in a ring-roll mill.
In case of the second alternative of the proposition according to the invention (toothing carrier plates), similarly favourable conditions result in view of the forging and the hardening in case of a corresponding dimensioning of the respective plate thickness.
At first, the possibilities of development of the first alternative (toothing carrier casing) will now be treated. The toothing carrier casing can be joined at at least one end, preferably at both ends, to a end plate, in the central region of which the bearing journal or the connection portion is arranged . If end plates with one respective bearing journal or other connection portion are mounted on both ends, the same possibilites as to the bearing of the toothed wheel unit result as with the known full shaft constructions or hollow shaft constructions. Compared to the size of the entire toothed wheel unit, the end plates with bearing journals or other connection portions have a relatively small volume so that the problems on casting, forging and hardening and during operation due to centrifugal force and temperature stresses can be avoided.
Besides the two-plates-solution, one embodiment with only one end plate will nevertheless be discussed, because the creation of a cantilevered toothed wheel unit should not be excluded. Furthermore, it is not to be excluded that a toothing carrier casing is produced with an end plate on one end and is joined to an end plate on the other open end. With such a solution, the problems of casting, forging and hardening and during operation due to centrifugal force and heat stresses are also reduced to a large extent.
The end plate can be produced integrally with the bearing journal or other connection portion. Due to the geometry of a component consisting of an end plate and a bearing journal, easier conditions for casting, forging and hardening result.
The end platexe2x80x94viewed in a sectional plane containing the axis of the toothed wheel unitxe2x80x94has walls which extend deviating from a plane normal to the axis in the radial intermediate region between the outer diameter of the bearing journal or connection portion and the inner diameter of the toothing carrier casing. This measure is in particular taken in view of the neutralization of centrifugal forces. If a toothing carrier casing, i. e. a hollow tube, is provided according to the invention, it is naturally more sensitive to centrifugal forces occurring in rapid movement operation than a toothed wheel unit of the full shaft construction. On the other hand, end plates, which are closed in their center or which extend radially inward to a smaller diameter corresponding to the diameter of the bearing journal, are naturally less sensitive to centrifugal forces, i. e. they are less subjected to a radial increase of diameter under the influence of centrifugal forces. This could lead to stresses in the connection zone between the end plate and the toothing carrier casing. One possibility to find a remedy is to provide for an elastic behaviour of the end plate under the influence of a centrifugal force. Such an elastic behaviour can be obtained by shaping the end plate such that asxe2x80x94viewed in a sectional plane containing the axis of the toothed wheel unitxe2x80x94the wall shape and the mass distribution of the end plate are such that radial displacements of the connection region on the end plate side and of the connection region on the toothing carrier side due to centrifugal forces are approximated to one another irrespective of the connection of the two connection regions.
A development is possible in whichxe2x80x94viewed in a sectional plane containing the axis of the toothed wheel unitxe2x80x94the wall shape of the end plate has a convex curvature towards the inside of the toothing carrier casing in the radial region between the outer diameter of the bearing journal or connection portion on the one hand and the toothing carrier casing, on the other hand.
Another possibility to limit stresses in the connection region between the end plate and the toothing carrier casing caused by centrifugal forces is to provide the end plate with a compensation mass which radially exceeds the zone of connection with the toothing carrier casing. Due to the compensation mass, the end plate, in spite of its material extending further radially inward compared with the toothing carrier casing, suffers a similar or equal expansion under the influence of radial forces as the toothing carrier casing.
Several possibilities exist to join the end plate with the toothing carrier casing; according to claim 8, it is possible that the end plate is welded to the toothing carrier casing at the zone of connection; it is furthermore possible that the end plate is in a torque transmitting or/and centering form-fit connection or in a force transmitting connection with the toothing carrier casing at the zone of connection, a serration being possible as an interesting version of a torque transmitting and centering form-fit connection.
The axial connection, which possibly also serves for centering, can be obtained in such a manner that the end plate is axially tensioned towards the toothing carrier casing at the zone of connection, axial tensioning means being provided in the radial region of the zone of connection or radially inside the zone of connection, preferably in the axis region of the toothed wheel unit for the axial tensioning of the end plate with respect to the toothing carrier casing.
Besides the already mentioned serration, other centering means can be provided between the end plate and the toothing carrier casing
With all kinds of connection between the end plate and the toothing carrier casing, attention has to be paid that the connection itself does not lead to the development of new harmful stresses.
In general, case-hardened steel can be used for the end plates and the bearing journals joined to them, if desired. However, it is also possible to use tempering steel for the end plates, because the demands on the tooth engagement behaviour of the toothed ring, which require the use of case-hardened steel for the toothing carrier casing, do in general not exist for the end plates and the bearing journals.
In principle it is also possible to produce the proposed toothing carrier casing of highly quenched and subsequently drawn tempering or nitriding steel. Also in this case, this structure yields a higher precision of the stresses.
Like in the prior art according to the above-cited BHS-publication with the masthead H-3/1-87, it is possible according to claim 15 that at least one of two bearing journals or connection portions is made hollow and that a shaft associated to the toothed wheel unit is guided through the hollow bearing journal or connection portion and is in torque transmitting connection with the toothed wheel unit inside the toothing carrier casing or in the region of an opposing bearing journal or connection portion. In this way, a possibility to influence the stiffness against torsion of the gear train by an extension of the associated shaft is provided also in the toothed wheel unit according to the invention. The BHS-publication H-3/1-87 allows to clearly recognize the difference with respect to the basic idea of the present invention also inasmuch as on page 11 of the publication a hollow shaft structure is shown, in which the bearing journals are produced integrally with the plate body so that the problems of casting, forging and hardening persist in spite of the reboring, whereas in addition the operative stresses due to centrifugal force and heat stresses are considerably higher, especially in the region of the bore.
The proposition according to the invention can be applied independent of the arrangement of the external toothing; the toothing can e.g. be arranged in form of a spur toothing, a helical toothing and a double helical toothing.
The thin-walled structure of the toothed wheel unit according to the invention allows to provide cooling surfaces in the toothing carrier close to the external toothing, said cooling surfaces being used for heat exchange contact with a coolant and/or for conducting said coolant. This is important, as especially in rapidly moving toothed wheel units, large heat production occurs in the region of engagement with the counter-toothing, which can lead to heat gradients inside the toothed wheel unit. If one manages to dissipate this heat, the course of the gradient becomes flatter. This means lower resulting heat stresses and in particular a considerably more stable behaviour of the toothing contact pattern. The latter leads to a more uniform load of the teeth and thus either to a higher loading capacity or to a higher tooth security.
The possibility exists in the embodiment with a toothing carrier casing that said cooling surfaces used for heat exchange contact and/or for conducting said coolant comprise a cooling casing, which is arranged radially inside the toothing carrier casing. Various coolant flow patterns can be produced adapted to the heat production profile. It is e.g. possible that the coolant conducting cooling surfaces cause a coolant flow, which is essentially parallel to the axis direction of the toothed wheel unit; according to claim 20, it is furthermore possible that the coolant conducting cooling surfaces cause a coolant flow with a helical course around the axis of the toothed wheel unit; it is furthermore possible that the coolant conducting cooling surfaces cause a coolant flow, which passes at least once continuously all over the axial length of the external toothing.
Furthermore, various combinations of inflow and discharge are possible, again adapted to the heat production profile. It is e.g. possible that the coolant conducting cooling surfaces cause a coolant flow, which passes from both axial ends of the external toothing to an axial center region of the external toothing; it is furthermore possible that the coolant conducting cooling surfaces cause a coolant flow, which passes from an axial center region of the external toothing to both axial ends of the external toothing; it is furthermore possible that the coolant flow is guided through the toothing carrier casing towards the surrounding region of the external toothing in the axially central region of the external toothing, preferably in the region of an axially central interruption of the external toothing.
A simple possibility to guide the coolant consists in the fact that the cooling casing is provided with spacer or/and coolant conducting ribs on its outer surface directed towards the inner circumferential surface of the toothing carrier casing. However, it is additionally and/or alternatively possible that ribs are arranged at the inside of the toothing carrier casing. For reasons of production, the latter possibility is, however, less preferred.
Depending on the respective desired coolant flow patterns, it is possible that the ribs extend essentially parallel to the axis of the toothed wheel unit, or it is possible that the ribs extend helically around the axis of the toothed wheel unit.
It is possible and advantageous, but not necessary, that the toothing carrier casing is formed by at least one single-piece tube section. The desired effect of the invention is obtained to a high degree, if in case of tube sections having external diameters (toothing diameters) of 900 mm and more the radial thickness of the tube section is less than 200 mm, preferably less than 150 mm, the radial thickness being reduceable in case of tube sections having external diameters (toothing diameters) of less than 900 mm, the ratio of reduction corresponding to the ratio of the respective external diameter:900 mm. In other words, it is advantageous according to the invention, if according to claim 30, the radial thickness of the tube section is less than 30% of the tube section external diameter (toothing diameter), preferably less than 20%.
Furthermore, the geometrical relations can also be represented in such a way that, the radial thickness of the tube section less the toothing height is more than four times the real pitch module of the toothing, preferably more than eight times the real pitch module of the toothing. The wall thickness instructions for the toothing carrier casing given above also are chosen such that the connections with end plates can be made without the occurrence of additional dangerous stresses.
In what follows, the second alternative of the invention will be discussed, which aims at an axial subdivision of the toothing carrier.
Two or more toothing carrier plates can follow one another, the axial thickness of the toothing carrier plates being less than 200 mm, preferably less than 150 mm. Particularly appropriate possibilities for the access of heat treatment and other treatments to the total volume of the toothing carrier are obtained, if the toothing carrier casing is formed by toothing carrier ring plates following each other in the axial direction and indirectly or directly joined to one another.
A subdivision into axial sections is possible, even if a continuous toothing over several plates is to be provided. In this case, however, one has to pay attention to a particularly exact mutual centering of the plates by centering means, like e.g. serrations, in order to avoid considerably increased tooth wear at the contact zones in the toothing region.
The subdivision into axially adjacent plates is particularly advantageous, if the external toothing itself is subdivided in axial direction one or more times for reasons of toothing characteristics, as e.g. in the case of double helical toothings. One will then make the subdivision of the plates coincide with the axial subdivision of the external toothing length.
The second alternative of the invention (toothing carrier plates axially following one another) can also be realized in such a way that the toothed wheel unit comprises a shaft with a radial flange and that a respective toothing carrier ring plate is mounted on each side of the radial flange. With such a structure, too, relatively uniform and short distances result for the action of a treatment onto the total volume of the parts forming the toothed wheel unit. It is possible that the toothing carrier ring plates are connected with the radial flange by a respective torque transmitting or/and centering form-fit connection and axial tensioning means.
On ring-shaped moulding of toothing carrier plates axially following one another, the dimensional values according to claim 38 are preferably kept to in such a way that the radial thickness of the toothing carrier ring plates and the axial thickness of the toothing carrier ring plates are less than 300 mm, preferably less than 200 mm.
The moulding of the toothed wheel unit according to the invention allows to mount temperature sensor elements, preferably thermoelements, next to the toothing, preferably at the inner circumferential surface of the toothing carrier casing. This is not only advantageous as it allows to indicate dangerous conditions. Instead, it is also possible, e.g. if cooling by means of a coolant is provided, to control the coolant throughput depending on the measured temperatures.
The arrangement according to the invention can be developed in such a way that the toothed wheel unit comprises at least one annular pressure comb and/or one pressure comb engagement ring, preferably as a part of an end plate or a compensation mass mounted to an end plate.